This invention is concerned with the improvement of a tool having an air driven motor with an overspeed safety mechanism to ensure the air motor from developing a speed beyond a predetermined safe limit.
The overspeed mechanism is especially suited for high speed surface grinding tools because of the dangers presented by such tools should they, for some reason, overspeed without control. Such tools are usually equipped with a speed control governor which functions to maintain the operation of the motor within the safe limits of a predetermined speed rate. However, there is the possibility of the governor malfunctioning due to entry of dirt, breaking of parts, accumulation of rust, and other reasons, so that the motor could develop an undesirable overspeed condition.
The safety mechanism of the present invention may be used in rotary tools without speed regulating governors as well as in tools having such governors. In either case, the safety mechanism is intended to stop flow of operating air to the motor when a predetermined overspeed condition develops. When incorporated in a tool having a speed regulating governor, the latter functions primarily to maintain the speed of the motor substantially at a predetermined rate; and, when the motor develops a predetermined higher level of speed or overspeed condition, the safety mechanism functions automatically to shut off air flow to the motor and thus causes the latter to stop.
The safety mechanism is so designed that when it is caused to operate, the tool becomes disabled and cannot be effectively further operated until the tool is taken apart and the safety mechanism reset. Normally, the safety mechanism would not be reset until after an inspection of the tool as to the cause for the overspeeding had first been made and after any necessary repairs have been made.
In accordance with the invention, there is provided for incorporation in a tool having an air driven rotary motor centrifugally operable air shut-off control mechanism comprising an annular body member mounted to a rotor shaft and having a radially extending hole in its periphery in which a heavy ball valve is releasably retained by the free ends of a curved spring mounted to the periphery of the body member, the ball valve being responsive to centrifugal forces developed by an overspeed condition of the motor to escape tangently from between the ends of the spring and seat in a pocket to seal an inlet port so as to cut off further flow of operating air to the motor.